1. Field Of The Invention
The present invention relates to interference cancellation systems and radio systems, and more particularly relates to interference cancellation systems operable with multiple radios.
2. Description Of The Prior Art
To facilitate an understanding of the present invention, the following description of a basic interference cancellation system is provided.
FIG. 1 shows a conventional interference cancellation system for operation with two radio systems having radios 1,2. It is assumed that first radio 1 is in the transmitting mode and the second radio 2 is in the receiving mode. Each of radio 1 and radio 2 is connected to its own antenna 6,8 for transmitting and receiving signals, each antenna 6,8 being shown connected to its respective radio by a transmission line 10,12. The signal transmitted by radio 1 is an interfering signal which is received by radio 2 through its respective antenna 8 and interferes with the ability of radio 2 to receive a desired signal. Hence, an interference cancellation system is employed and is connected between the first radio system and the second radio system to cancel the interfering signal component in the signal received by radio 2.
The conventional interference cancellation system includes a first directional coupler 14, referred to as a reference coupler, which is coupled to the transmission line 10 of the first radio system to obtain a sample of the transmitted signal (i.e., the interfering signal). This sample of the interfering signal is commonly referred to as the reference signal. Similarly, a second directional coupler 16, which is commonly referred to as an error coupler, is connected to the transmission line 12 of the second radio system to sample the received signal on the transmission line, which received signal includes an interfering signal component and a desired signal component. This sample of the received signal is commonly referred to as an error signal.
The interference cancellation system also includes a synchronous phase detector 18 and a vector modulator 20 (also known as a signal controller). A portion of the reference signal is provided to the reference input port of the synchronous phase detector 18 using another directional coupler 22, and the error signal is provided to the error input port of the synchronous detector. The synchronous detector 18 compares the two signals, and generates two detector output signals which vary in accordance with the differences in amplitude and phase between the reference signal and the error signal.
Each of the detector output signals may be provided to an integrator/amplifier 24, which will provide time varying, DC control signals which vary in response to the detector output signals. These control signals are provided to the vector modulator 20.
The vector modulator 20 receives a portion of the reference signal and adjusts the amplitude and phase of the reference signal in response to the control signals it receives from the synchronous detector (via the integrators/amplifiers 24). The vector modulator 20 generates a cancellation signal which is injected onto the transmission line of the second radio system using a fourth directional coupler 26, also known as a summing coupler. The cancellation signal has an equal amplitude but an opposite phase to that of the interfering signal component received by the second radio system, thereby cancelling the interfering signal component from the received signal.
Thus, the interference cancellation system automatically and continuously maintains the amplitude and phase of the cancellation signal for maximum cancellation of the interfering signal component from the signal received by second radio system. This conventional system is described in
in U.S. Pat. No. 5,152,010 entitled "Highly Directive Radio Receiver Employing Relatively Small Antennas", which issued on Sep. 29, 1992 to Ashok K. Talwar, one of the inventors herein, the disclosure of which is incorporated herein by reference.
The effectiveness of the interference cancellation system described above and shown in FIG. 1 is degraded when the isolation between the two antennas 6,8 is low. This occurs with collocated radios, and is especially a significant problem when the antennas of the radio systems must be closely situated to each other, such as when they are mounted on a small platform, such as an aircraft, with very little room for providing spacing between the antennas. The coupling between the two antennas may be as much as 6 dB. Because of this high coupling and low isolation between the two antennas, the interference caused by the power radiated by the transmitting radio degrades the performance of the receiving radio located on the same platform. The conventional method of interference cancellation may not be effectively employed because the power of the reference signal required to cancel the large interfering signal component in the received signal after overcoming reference and summing coupler losses and losses in the vector modulator may not be available.
For example, the vector modulator 20 (i.e., signal controller) may have as much 10 dB in losses, and each of the reference and summing couplers may have 10 dB in losses associated with them. Thus, there may be 30 dB in losses in the reference signal path of the interference cancellation system, while only 6 dB between the antennas of the transmitting radio system and the receiving radio system. Thus, the interfering signal component in the received signal of radio 2 might always be stronger than the reference signal tapped from the transmitting radio system after passing through the high losses associated with the reference signal path of the interference cancellation system. This problem is especially acute with collocated radios with their antennas closely spaced together, such as on a small platform. Thus, when the isolation between the two antennas is low, the coupled reference power may be insufficient to provide cancellation.
One attempt to overcome this problem is to take additional power from the transmitting radio and couple it through the reference signal path so that the reference signal and the cancellation signal have sufficient power to cancel the interference signal component in the received signal of radio 2. A problem with this approach is that now the transmitted power is decreased and may be much less than the coupled reference signal power.
A second approach to resolving this problem is to add an attenuator to the receiver transmission line 12 to introduce excessive loss at the output of the receiving antenna 8. This, of course, reduces the receive sensitivity of radio 2. With either approach, communication link performance is greatly degraded.
An amplifier may also be added in the reference signal path either before or after the vector modulator 20. Such an amplifier would have to have high power to overcome the losses in the reference signal path. This approach to the problem may result in distortion to the cancellation signal and the introduction of more noise to the system.
Another major disadvantage of having multiple radios collocated on a small platform, each having its own antenna, is that performance of the radios is degraded by the proximity of the antennas to each other, which causes the antenna pattern of each antenna to be distorted. This will reduce the range capabilities of the communication link between the radios on the small platform and a remote receiver or transmitter.